The pressure on Ke$ha to be thinner – which led to rehab last week for eating disorders – didn't just come from the singer herself, her mother claims.

Pebe Sebert tells PEOPLE that her 26-year-old daughter's advisers, including her producer Dr. Luke, made her feel like she desperately needed to lose weight. A spokesman for the producer vehemently denies this claim.

"Dr. Luke had been telling her how she had to get in shape and lose weight and all this stuff. She was exercising and dieting and ultimately doing everything she could, but not getting thin fast enough," says Sebert. "And that's when she first became bulimic."

"Dr. Luke told a friend of ours he thought she looked like a refrigerator on her latest video, and could she please try to control her weight," Ke$ha's mom adds. "After she went on a 14-day juice cleanse, he said, 'Thank God you finally got rid of the weight! We've all been talking about it!' So she felt like she couldn't trust anyone."

The turning point, says Sebert, came just before the holidays: "Right before Christmas she had a breakdown, and called me from the gas station after she'd thrown up everything, and admitted how much she'd been doing it."

But in an exclusive statement via his rep, Dr. Luke denied Sebert's allegations to PEOPLE.

"It is unfortunate that at a time Ke$ha is suffering immensely and trying to heal at a treatment center, her mother is taking this approach with the media. I give Ke$ha my utmost support and well wishes for a quick return to health."

Upon entering rehab, Ke$ha said in a statement: "I'm a crusader for being yourself and loving yourself but I've found it hard to practice. I'll be unavailable for the next 30 days, seeking treatment for my eating disorder … to learn to love myself again. Exactly as I am."